Feature/track subtr

[tagirahmad]

What is the purpose of this pull request?

Solves issue by implementing SQL subtransactions detecting and reacting on it depending on specified configuration.

What changes did you make? (overview)

You can use feature like:

Isolator.configure do |config|
  # When to report nested substransactions depending on the depth (32 is the default)
  config.substransactions_depth_threshold = 32
   # Specify a custom logger to log offenses
  config.report_subtransactions = :log # or :exception or :notifier for uniform_notifier
end

Is there anything you'd like reviewers to focus on?

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated a documentation

[palkan]

Thanks! We introduced some new APIs and concepts since you've opened the PR, so I would like to upgrade the implementation as follows:

• The functionality MUST be implemented as a plugin (see example).
• No need to add custom reporting, just a single configuration parameter to specify the limit (and activate the feature).